490 Absorption Spectrophotometry /26 : 3 



spectrophotometer has several components. These include the light 

 source, the monochromator, the sample holder, the detector, and the 

 associated electronic amplifiers and recorders. These are discussed 

 briefly on the following pages. 



A. Light Sources 



For spectrophotometers operated in the visible and ultraviolet regions, 

 light sources are usually either gas discharge tubes or heated filaments. 

 The discharge tubes give a line spectrum at long wavelengths and a 

 continuous spectrum at the shorter ultraviolet wavelengths. The heated 

 filaments emit continuous light from the "near" infrared region to the 

 "near" ultraviolet region. 



For most studies, a continuous spectrum is desired from the light 

 source. This makes it possible to study absorption spectra as a con- 

 tinuous function of wavelength. For the excitation of fluorescence and 

 for the calibration of monochromators, a line spectrum is more useful. 

 Usually, it is not possible to have one source of light which is satisfactory 

 over both the visible and the ultraviolet spectra. Most characteristic 

 absorption spectra are measured between 200 and 1,000 m/x. 



In the infrared regions of the spectrum, still other light sources are 

 needed. Generally, some form of hot glowing object is used, the visible 

 rays being filtered off from the infrared. Nernst glowers of rare earth 

 oxides and Globars of carborundum are the most frequently used 

 infrared sources. 



In any wavelength region, the electrical power source operating the 

 light must be carefully stabilized. Otherwise, fluctuations in light 

 intensity due to the changes in the electrical power may be greater than 

 the differences due to the absorption being measured. This is illus- 

 trated forcefully in the case of the incandescent filament. The power 

 delivered to the filament is roughly proportional to the square of the 

 applied electrical voltage. The temperature of the filament will vary 

 almost proportionally to the power consumed. The light emitted, 

 however, is proportional to the fourth power of the absolute temperature, 

 and hence, to the eighth power of the voltage. Thus, if the voltage is 

 represented by V and the light intensity emitted by / 



I oz {V) 8 



If V changes from V to V + A V, I changes from I to I + A/. The 

 apparent optical density change AZ) due to the change in / will be 



A _ . I +M 1 A/ 



[ o 



